Missing multipart boundary in the content type header

BlobsWriter does not write the multipart boundary in the content type header.

It should produce something like:
Content-Type: multipart/mixed; boundary="gc0p4Jq0M:2Yt08jU534c0p"

But it only returns:
Content-Type: multipart/mixed;

To reproduce call an operation that returns a multipart response (like Blob.toPDF) and check the response header 'Content-Type':

curl -v -X POST 'http://demo.nuxeo.com//nuxeo/site/automation/Blob.ToPDF' -u Administrator:Administrator  -F'request={}' -F'test.odt=@test.odt;type=application/vnd.oasis.opendocument.text' -F'test2.odt=@test.odt;type=application/vnd.oasis.opendocument.text'